During the past year we have continued to evaluate various aspects of the host-immune response so that more effective immunotherapeutic strategies might be planned. In the course of those investigations it has been ascertained that bone marrow from animals with a tumor present up to about two weeks produced a significantly greater number of macrophage colonies in-vitro than did appropriate controls. Moreover, it was observed that the administration of BCG to two tumor bearing animals enhanced that response over what was observed when BCG was given to normal animals alone. In a series of investigations bone marrow cells from tumor bearing animals were employed in the treatment of lung metastases. No consistently beneficial effect was observed in the model employed. Experiments are planned for the coming year which will be directed toward elaboration of findings concerning macrophage colony production by bone marrow cells and will be aimed at determining whether (1) removal of a tumor affects colony production, (2) tumor removal affects the BCG response indicating that continued presence of antigen is required for the response, (3) the route of administration of BCG influences the response, (4) C. parvum is as effective as BCG in stimulating bone marrow, and (5) splenectomy and/or RLN excision influences the phenomenon. Ultimately, the in-vitro findings will be related to in-vivo host immunity as determined by growth of a tumor following a tumor cell challenge. Should immune enhancement be demonstrated, metastases in animals will be treated by that program which has been demonstrated to most effectively stimulate bone marrow cells to produce macrophage colonies in-vitro.